mongoose populate 3 levels deep

gistfile1.js

var Blogpost = new Schema({
	story: 'String',
	comments: [{type: ObjectId, ref:'comment', unique: true}]
});

var Comment = new Schema({
	comment: 'String',
	upvotes: [{type: ObjectId, ref:'user', unique: true}]
});

var User = new Schema({
	name: 'String',
	friends: [{type: ObjectId, ref:'user'}]
});




db.model('blogpost', Blogpost);
db.model('comment', Comment);
db.model('user', User);

db.models.blogpost.find({}).populate('comment').exec(function(err, posts){
	db.models.comment.populate(posts.comment, 'upvotes', function(err, comments){
		// populates upvotes with user objects
		db.models.user(posts.comment.upvotes, 'friends', function(err, friends){
			// doesnt populate friends;
		});
	});
});